1. Field of the Invention
The present invention relates to a fan for generating an air flow, in which a rotor of a motor is arranged in an impeller cup.
2. Description of the Related Art
Electronic devices such as personal computers are equipped with fans for generating air flow in order to minimize the temperature increase in the electronic devices. The air is used for cooling a particular component and/or discharging heats generated in the electronic devices.
Those fans usually generate an air flow by rotating an impeller having a plurality of blades on its outer side surface with a rotational force generated by a motor. In most of those fans, a rotor of the motor is arranged inside an impeller cup of the impeller.
The structure of the typical fan is described in more detail. The impeller of the fan includes the impeller cup which is generally cylindrical. The blades are arranged outside the impeller cup in a circumferential direction of the impeller cup. Inside the impeller cup is arranged the motor. The motor mainly includes: a rotor yoke which is cylindrical; a cylindrical rotor magnet attached to an inner side surface of the rotor yoke; and a stator arranged inside the rotor yoke to oppose the rotor magnet. The rotor yoke is supported in a rotatable manner relative to the stator. When power is supplied to coil windings of the stator, a magnetic field generated by the stator interacts with magnetic poles of the rotor magnet, thereby generating a rotational force which rotates the rotor magnet relative to the stator. The rotor magnet rotates together with the rotor yoke about a center axis of the motor. The rotor yoke and the rotor magnet form together a rotor portion which is fitted at its outer side surface to the inner side surface of the impeller cup coaxially with each other. That is, the rotor yoke is fitted to the inner surface of the impeller cup. With this configuration, the impeller rotates together with the rotor yoke with the rotational force generated by the motor, i.e., the force rotating the rotor yoke about the center axis. The blades of the impeller are turned about the center axis by the rotation of the impeller so as to generate an air flow which is used for cooling the inside of the electronic devices.
Improvement of the cooling performance of fans for use in electronic devices has been demanded recently in order to cool the inside of the electronic devices more efficiently. This demand is met by increasing the discharged amount of hot air from the inside of the casing of the electronic devices to the outside, for example. One exemplary technique for achieving this is to increase the amount of air being transmitted from fans.
When a surrounding wall of the impeller cup of the fan is arranged so as to move away from its center axis from an air-inlet side of the fan to an air-outlet side, i.e., the impeller cup is formed such that its outer diameter increases toward the air-outlet side, for example, air intake resistance is reduced. Thus, the amount of air transmitted from the fan is increased. In this case, however, a fastening force of the impeller cup applied to the rotor yoke when the rotor yoke is press-fitted to the inside of the impeller cup may be insufficient. More specifically, since the rotor yoke usually has a cylindrical outer side surface, the fastening force of the impeller cup acts in a direction that is generally perpendicular to the surrounding wall of the impeller cup which is at an angle to the center axis. Therefore, a radial component, i.e., a component in a radial direction that is perpendicular to the center axis, which is required for fastening the rotor yoke, may be insufficient. The radial component of the fastening force can be increased by reducing the inner diameter of the surrounding wall of the impeller cup. However, in this case, stress beyond design limitations is applied to the surrounding wall when the rotor yoke is press-fitted to the impeller cup, causing deformation or breakage of the impeller cup.